The purpose of this investigation is to find the reliability of the measurement of pulmonary tissue volume by the rebreathing method. A non-invasive, rapid technique to measure tissue volume by using soluble inert gases has been described; this method would be very useful in following the course of pulmonary edema during experimental interventions as well as possibly clinically. However, since the method relies on ventilation to deliver the test gas, it suffers potentially from maldistribution of ventilation and mismatching of ventilation and perfusion. Indeed, this limitation has been shown to exist theoretically. We plan to measure pulmonary tissue volume in animals whose distribution of ventilation and perfusion has been disrupted. The distribution of gas and blood is also measured during the experiment by using the multiple inert gas method. The tissue volume measurement is compared with the wet weight of the lung to find its reliability. The measurements will be made both in animals with normal lung weight and in animals with pulmonary edema. Thus we should conclude how much maldistribution of ventilation and perfusion can be present before the measurement of pulmonary tissue volume becomes unreliable.